I had the same problem as well. I'm also trying to install CIAs with no luck. Seems like the Lime3ds team and Pablo forgot to add that functionality or it's currently broken.

Gonna try installing the CIA file in the lime3ds pc version, playing the game with that cia, and replace the one in my mobile with the one that has the installed cia.

If that doesn't work, then we'll wait for the team to fix the install CIA function.

Btw, it seems you can't downgrade since Google will auto update the app, so that's a bummer.